Conventional surface topography interferometric microscopes for profiling the surface of a microstructure with respect to an X-axis, a Y-axis and a Z-axis of a Cartesian coordinate system are static measurement means that can be adapted for various applications, including: for measuring the surface roughness and flatness of a wafer, for measuring depth of a lease mark, for measuring coplanar as well as the size of the stud bump in a flip chip process, for measuring the size and height of spacers in a color filter of an LCD flat panel display, for measuring the surface profile of the endface of an optical fiber as well as that of a micro-optical element, and so forth. Recently, there are efforts trying to improve the conventional static surface topography interferometric microscope with dynamic measuring capability so that it is possible to use the aforesaid interference microscope with dynamic measuring capability in micro electromechanical system (MEMS) and micro optical electronic mechanical system (MOEMS) for inspecting an measuring the dynamic of devices and membranes.
The aforesaid efforts usually achieve the dynamic measurement by attaching a driving source with a fixed excitation frequency onto a working sample and the same time connecting a synchronization device to its light source for synchronizing the frequency of the light source with that of the driving source in a manner that a resonance frequency can be obtained manually by altering the frequency of a synchronization signal. However, For those wide-band driving source such as a micro cantilever submerged in water which is resonating by the affection of flowing water for enabling the micro cantilever driving source to emit a wide-band signal, they are not suitable for dynamic measurement since it is difficult to control the resonance of the driving source.
The most common vibration measurement is the vibration analysis performed by the use of a Laser Doppler Velocimetry (LDV). However, it is only useful for single-point measurement in a manner that when it is required to measure a two-dimensional vibration, a step-scan mode is adopted for scanning every single point of a mechanical structure in a point-by-point manner and thus acquiring the vibration mode thereof. As the aforesaid method is not only time consuming, but also can be easily affected by ambient environment during the measurement, it is unable to obtain the surface profile relating to a complete vibration mode in a real time manner.
Taking the Polytec vibrometer MSA400for example which is an improvement over the Mach-Zehnder interferometer, it is composed of a laser Doppler module, a white light interference module for static 3D profilometry and a strobed light displacement measuring module, and so on. For performing an out-of-plane displacement measurement, the MSA400 first utilizes its embedded software to mark optimal sampling points to be sampled as well as map out the optimal sampling path accordingly in advance in the area that is to be inspected, and thereafter uses its laser Doppler module to scan and analyze each and every sampling points within a specific frequency range for obtaining resonance frequencies in respective. As from each sampling point, it is able to obtain a corresponding resonance frequency set. Thus, by integrating those resonance frequency sets, a 3D profile of an out-of-plane displacement can be established. The aforesaid method of using a laser beam to scan a surface in a point-by-point manner is unique and patented by Polytec. However, as the aforesaid method is not a full-field measurement, it is unable to detect the defects that only affect on certain MEMS devices when they are resonating. Nevertheless, the aforesaid method is advantageous in its high band width that it can be adapted for inspecting an object as large as an automobile, or as small as a MEMS device.
Studying currently available technical reports and documents relating to 3D profiling method and system, it is noted that there are problems required to be solved which are: (1) although the laser Doppler scanning is able to provide sufficient identification in wide-band resonance frequency detection, the laser energy that it is used for detection is possible to cause damage to a micro device which it is inspecting; (2) as laser source is usually very expensive and requires complex optical path that is not easy to establish, it is difficult to apply the aforesaid laser Doppler scanning in common microscopic system.